In the design of complementary metal-oxide semiconductor (CMOS) integrated circuits, differential amplifiers are used for various applications, such as digital or analog amplifiers, comparators, buffer stages, and the like. For example, in a digital integrated circuit (IC), input receivers on the chip may employ differential amplifiers for buffering digital input signals to make input signal levels from external sources compatible with internal switching levels. Conventional differential amplifier circuits do not have hysteresis. That is, changes in the output of the differential amplifier are synchronized with changes in the input (i.e., the output of the amplifier does not lag the input of the amplifier). Without hysteresis, small undesired variations in input voltage (e.g., due to noise) can cause the output voltage of the amplifier to change in a non-ideal fashion.
Consider a digital input buffer where one input of the differential amplifier is coupled to a reference voltage, and the other input of the differential amplifier is configured to receive a digital input signal. In some circumstances, the two input voltages may be fairly close in value. For example, when the digital input is floating, the logic level of the digital input is typically pulled to the reference voltage by a termination resistor. Since the output of the differential amplifier changes state when there is any difference between the two input voltages, variations on either input due to noise can cause undesired switching at the output Therefore, it is desirable to compensate for noise on the inputs of a differential amplifier. It is further desirable for an input receiver in an IC to compensate for noise, rather than forcing the user of the IC to compensate for such noise at a board-level.
Accordingly, there exists a need in the art for a method and apparatus for providing a differential amplifier with hysteresis.